1. Field of the Invention
The present invention relates generally to communications systems, and more particularly to an autonomous battery-free microwave frequency communication device.
2. Description of the Related Art
A conventional battery-free contact-less wireless communication device is known which is based upon the ISO/IEC (International Organization for Standardization/International Electrotechnical Commission) 14443 near-field communication specification, which uses a relatively-low carrier frequency of 13.56 megaHertz (MHz) and relatively-low data rates of up to 848 kilobits per second (kbps) and involves a battery-powered reader referred to as a “Proximity Coupled Device” or PCD and a battery-free, energy-harvesting “tag” referred to as a “Proximity Integrated Circuit Card” or PICC. The power used to transmit the data read/write requests from the PCD to the PICC is inductively coupled from PCD to PICC at a range of approximately 20 centimeters (cm) or less which is within the “near field” of the PCD. The PICC communicates responses to the PCD by modulating a load according to backscatter communications. In accordance with backscatter communications, there is no active modulation of a signal transmitted from PICC to PCD, there is no generation of an independent carrier by the PICC transmitter, and the PICC must be in the near field of the PCD. The near field is necessary to establish magnetic coupling in which communication is based on induced current. These systems commonly have a simple integrated state machine and associated memory and are currently used in some contact-less credit and debit cards as well as identification cards.
Certain standards that cover near-field communications (NFC) with passive tags include, but are not limited to, ISO/IEC 14443 and 15693 (13.56 MHz carrier frequency), ISO/IEC 18000 (135 kiloHertz (kHz), 13.56 MHz, 2.45 gigaHertz (GHz), 860-960 MHz, and 433 MHz), ISO/IEC 18092 and 21481. ISO 18000-4, in particular, uses the 2.4-2.5 GHz band and has an option for microwave-frequency communication with a passive tag using backscattering.
FIG. 1 is a figurative and schematic diagram illustrating a conventional Radio Frequency Identification (RFID) near-field communication system 100 with an active RFID reader 101 and a passive RFID tag 103 that responds to the active RFID reader 101 via backscattering, very much like a radar illuminating a target. The active RFID reader 101 includes a magnetic loop antenna 105 which is placed in close proximity with a magnetic loop antenna 107 of the passive RFID tag 103 to establish a magnetic field 106. The passive RFID tag 103 further includes a shunt capacitance CTUNE, a switch SW, a full-wave rectifier 109 and a storage capacitor CS coupled to the magnetic loop antenna 107, in which CS develops a supply voltage VS for providing power to an RFID tag integrated circuit (IC) 111. The switch SW includes a series resistance RSW (which may or may not be a separate physical resistor, but may instead represent the series resistance of the switch SW). The RFID tag IC 111 is shown including a receive (RX) detector 113, control logic 115, transmit (TX) switch control 117 and memory 119.
In this RFID system, the active RFID reader 101 operates as an interrogator which develops the magnetic field 106 to provide power and which further modulates the magnetic field 106 to enable communication with tags that are within their range, such as the passive RFID tag 103. When the active RFID reader 101 is placed in close proximity with the passive RFID tag 103, the magnetic loop antenna 107 develops inductive current which is converted to voltage across CS for providing power to the RF tag IC 111. The active RFID reader 101 further modulates the magnetic field 106 to send data, which is detected by the RX detector 113. Such modulation may be according to any suitable form, such as amplitude modulation (AM) (e.g., on-off key AM), frequency modulation (FM) or phase modulation (PM). The control logic 115 retrieves the data and may provide a response by controlling the switch SW via the TX switch control 117. During the time that the passive RFID tag 103 communicates back to the active RFID reader 101, the active RFID reader 101 broadcasts a steady radio frequency (RF) power level via the magnetic field 106, and the passive RFID tag 103 modulates the impedance of its RF load attached to the magnetic loop antenna 107 by adjusting its reflectivity by controlling the switch SW coupled with other passive components, such as CTUNE. The active RFID reader 101 then receives the data back from the passive RFID tag 103 as a variation in reflection of its transmitted power.
In this system, the passive RFID tag 103 can only send data to the nearby interrogator/reader, e.g., the active RFID reader 101, and the active RFID reader 101 sends data (by induced current) to the passive RFID tag 103. The passive RFID tag 103 sends data back to the active RFID reader 101 only while it broadcasts energy (e.g., while sending an un-modulated carrier signal via the magnetic field 106). The passive RFID tag 103 does not store energy for later use, and it does not generate its own RF carrier. Furthermore, the magnetic loop antennas 105 and 107 are typically rather large and are not commonly available for many types of devices, such as cellular phones or smart phones and the like. The active RFID reader 101, for example, is typically a tablet or hand scanner or the like particularly configured for RFID tag communications.
The conventional RFID tag communication systems, such as the communication system 100, have several disadvantages. The disadvantages include, for example, the need to have a relatively-large antenna to obtain sufficient energy-harvesting efficiency for the low carrier frequency (long wavelength of over 22 meters) and the lack of available reader interfaces in common devices like mobile phones and portable computers and the like. The conventional RFID tag communication systems operate in lower frequency ranges, such as tens of MHz, and operate at relatively low data rates, such as less than 1 megabit per second (Mbps).
Other systems, e.g., using 902-928 MHz for ultra high frequency (UHF) RFID harvest energy but also use backscatter communications. One potential advantage of such devices is that they operate using microwave frequencies. As used herein, microwave frequencies are within the range of about 300 MHz to about 300 GHz. Microwave frequency communications enable the use of relatively small antennas (<2 cm on a side) for increased energy-harvesting efficiency. The disadvantage of 902-928 MHz UHF systems, however, is that they also are not integrated into common devices like mobile phones and portable computers and the like. The common devices typically use Bluetooth or WiFi (802.11) technology, which are already integrated into cellular telephone handsets.
It is desired to provide RFID-type communications using battery-free passive tags that are able to communicate with common devices, such as those which operate using standard microwave frequency communications (e.g., Wi-Fi, Bluetooth, etc.).